Force and Rheology

Abstract

In this chapter we discuss the basics of dynamic descriptions of geodynamic processes. Dynamic descriptions, for example force balance estimates for orogens, provide a useful independent constraint on models based on field observations. Let us consider an example: A field geologist finds in a Precambrian shield folds and thrusts that he interprets to have formed as the consequence of crustal shortening. A detailed strain analysis shows that 80% shortening occurred and the geometry of shortening indicates that this resulted in fourfold thickening of the crust. He therefore further infers (using the principle of isostasy) that — at the time, a mountain range of some 15 km elevation existed above the metamorphic terrain. While this interpretation may be consistent with the field observations in the terrain, it has no independent test. In this example we could argue that we have no knowledge of any present day mountain range on this planet that is this high and that, therefore, this interpretation is unlikely. However, in many less obvious examples there are no direct analogies and the resulting models — albeit perfectly imaginable and fully consistent with field observations — are wrong. One way to provide an independent test of such models is to make a rough estimate of the involved force balance. In the next chapters we want to perform such estimates. In order to do so, it is necessary to commence with a brief repetition of the basics of stress and strain. However, this repetition remains brief and the interested reader is referred to a range of excellent text books on the subject. For example:

• Jaeger and Cook (1979) Fundamentals of Rock Mechanics.

• Twiss and Moores (1992) Structural geology.

• Pluijm and Marshack (1997) Earth Structure: An Introduction to Structural Geology and Tectonics.

• Ramsay and Huber (1983) Modern Structural Geology. Volume 1: Strain Analysis

• Ramsay and Huber (1987) Modern Structural Geology. Volume 2: Folds and Fractures

• Ramsay and Lisle (2000) Modern Structural Geology. Volume 3: Applications of Continuum Mechanics in Structural Geology

• Weijermars (1997) Principles of Rock Mechanics.